This is an old revision of the document!
The GTK version is now obsolete as no one is actively working on it.
Main window - Qt version this is the default GUI for the official Microsoft Windows builds. KDE 4/5 users may prefer it, too, since KDE 4/5 is based on Qt4/5.
Main window - GTK+ version this is the original Avidemux GUI, suitable for Linux/BSD systems with the GNOME, Xfce or LXDE desktop environments.